The present invention relates to a hydrogel forming material employed as a material for hydrogel having an excellent optical property, a high gas permeability and an excellent mechanical property.
Recently, a hydrophilic polymer has become popular with increase in application of polymer material to the field of medical industry. Especially, polymeric material which is not soluble in water and has a high water-absorbing property has been employed as materials of various films, including materials of films for medical use, catheters, canulas, water containing contact lenses, and the like.
With respect to the above hydrogel, the hydrogel has a tendency that the mechanical property suddenly lowers with increasing water content; and the higher gas permeability, the lower the transparency. Therefore, there has never been obtained a hydrogel which can sufficiently satisfy in optical property, gas permeability and mechanical property.
It higherto has been well-knonw that N-vinyllactam polymer has a high transparency and a high water solubility. Further it has been well-known that reinforcible ability of N-vinyllactam polymer is improved by copolymerizing N-vinyllactam and an alkyl ester of acrylic acid or methacrylic acid.
However, it is not easy to copolymerize N-vinyllactam and an acrylic acid or methacrylic acid. When N-vinyllactam is copolymerized with acrylic acid or methacrylic acid, it tends to give a mixture of 2 kinds of copolymers. That is to say, there are produced a copolymer of N-vinyllactam as a main component in which a few alkyl ester of acrylic acid or methacrylic acid is copolymerized, and a copolymer of alkyl ester of acrylic acid or methacrylic acid as a main component in which a few N-vinyllactam is copolymerized. When the copolymers contain water, they become cloudy and the mechanical properties of the copolymers are not satisfactory, because the copolymers tend to to phase-separate with each other.
There are proposed various methods of polymerization and cross-linking agents to dissolve the above-mentioned problems. For instance, N-vinyllactam and alkyl ester of acrylic acid or methacrylic acid are converted into a cross-linked copolymer of N-vinyllactam or a cross-linked copolymer of alkyl ester of acrylic acid or methacrylic acid by employing cross-linking agents which can easily cross-link each monomers to give a double cross-linked copolymer. And also, it is proposed a method employing cross-linking agents such as divinyl dicarboxylate, vinyl acrylate and vinyl methacrylate.
In accordance with the above-mentioned polymerization method, the polymerization form is complicated and further it is difficult to completely copolymerize the monomers and it is easy to elute an uncross-linked N-vinyllactam polymer. On the other hand, when divinyl dicarboxylate, vinyl acrylate or vinyl methacrylate is employed as a cross-linking agent, it is difficult to completely cross-link a polymer of N-vinyllactam. This difficulty is believed to occur because the vinyl group in the above-mentioned cross-linking agent has a low polymerizability in general to be apt to remain as an unpolymerized group, and therefore the vinyl group does not completely polymerize with N-vinyl group in N-vinyllactam.
It is an object of the present invention to provide a hydrogel having an excellent optical property, a high gas permeability and an excellent mechanical property.
A further object of the present invention is to provide a transparent hydrogel which is not cloudy at the time when the hydrogel is impregnated with water, that is to say, the hydrogel does not macroscopically occur phase-separation.